Seminal plasma contains very high concentrations of two potent immunomodulatory factors, TGF-β and PGE, which may promote female fertility by suppressing natural immune responses to sperm and the semi-allogeneic fetus. Humoral and cellular immunity to sperm and embryos have been associated with infertility and miscarriage (Kokcu et al. 2012; Hill 1995; Raghupathy 1997). The immunomodulatory roles of seminal TGF-β and PGE were recently reviewed in detail by Robertson (2013) and Doncel (2014). TGF-β is a ubiquitous pleiotropic immunomodulatory factor that has been implicated as a key factor in the modulation of host defense (Wahl et al. 2006; Gorelik and Flavell 2002). Its potent immunoregulatory activity became evident from studies in Tgfb1 and Tgfbr1 knockout mice (mice without TGF-β or the TGF-β receptor), which develop lethal inflammatory disorders early in life (Kulkarni et al. 1993; Marie et al. 2006; Shull et al. 1992). Three isoforms of TGF-β are present in human seminal plasma (TGF-β1, TGF-β2, and TGF-β3); they exist primarily in latent form until activation in the female genital tract by proteases and acid pH (Robertson et al. 2002). TGF-β induces stimulatory or inhibitory effects in human T cells depending on the T cell differentiation status and the stimulatory conditions (Oh and Li 2013). The first evidence that TGF-β plays a critical role at mucosal sites was provided by early studies on TGF-β signaling in the intestine showing that induction of regulatory T cells (Tregs) by TGF-β promoted immunological tolerance to food antigens by active control of innate and adaptive immune responses (Harrison and Powrie 2013). TGF-β has been suggested to be one of the major factors inducing immune tolerance in the female genital tract by inducing differentiation of Treg cells and suppressing the activity of natural killer cells (Robertson et al. 2002), thereby suppressing immunity to antigens expressed on sperm and the implanting embryo. TGF-β also upregulates the expression of proinflammatory cytokines and chemokines in epithelial cells from the female reproductive tract which could stimulate cell growth and angiogenesis to support embryo implantation (Sharkey et al. 2012a) and thus be beneficial to early events in pregnancy. PGE is another potent immunomodulatory factor in semen, capable of modulating immune functions on multiple levels (Quayle et al. 1989). The concentration of PGE in seminal plasma is several orders of magnitude higher than that in blood plasma, although there is a high degree of interindividual variation (Templeton et al. 1978). PGE suppresses macrophage and neutrophil function and the cytotoxic activity of T lymphocytes and natural killer cells and upregulates the inflammatory mediator Cox 2 in vaginal epithelial cells (Templeton et al. 1978). Kelley et al. showed that PGE exposure results in upregulated IL-10 and downregulated IL-12 production, shifting the T cell response from a Th1 (cell-mediated immunity (CMI) dominant) to Th2 (humoral immunity dominant) immune response (Kelley 1981). Soluble HLA-G is also present in semen and has been proposed to suppress adverse NK cell activity directed against invading cytotrophoblast (Rajagopalan et al. 2006). CD52g, a sperm-coating glycoprotein derived from the epididymis, may also play an important role in preventing antisperm immunity and infertility, although it can itself be a target of antisperm antibodies in some infertility patients (Norton et al. 2002). Seminal plasma also contains high concentrations of proinflammatory cytokines and chemokines that could affect fertility by recruiting and activating immune cells in the reproductive tissues and stimulating the production of factors that stimulate cell growth and angiogenesis (Politch et al. 2007). Specifically, seminal plasma has very high concentrations of IL-7, a hematopoietic growth factor that promotes the proliferation of lymphoid progenitors, B cell maturation, and T and NK cell survival (Fry and Mackall 2005), and three chemokines, SDF-1, MCP-1, and IL-8, which may recruit leukocytes to the insemination site to participate in immune defense and scavenger functions. The concentrations and ranges of principal immunomodulatory and proinflammatory factors in semen are provided in Table 7.1. Unpublished data from our laboratory indicate that TGF-β levels are decreased and IL-8 levels are significantly increased in semen from men with leukocytospermia (male genital inflammation) (Politch J, personal communication).

An exciting new area of research is the study of seminal exosomes. These highly abundant subcellular microvesicles, produced primarily by the prostate but also in the epididymis and seminal vesicles, are enriched in bioactive components including cytokines and small RNAs (miRNA, YRNA, and tRNA) and may play an important role in the fertilization and intercellular communication in the genital tract (Burden et al. 2006; Vojtech et al. 2014; Li et al. 2013). It is estimated that approximately one trillion exosomes are present in a human ejaculate. These small vesicles readily fuse with the plasma membrane of sperm and other cell types to deliver important signaling molecules. A number of immune-related mRNAs are targeted by miRNAs in seminal exosomes; whether miRNAs can be delivered by seminal exosomes in sufficient quantity to target genes and change cellular functions in the vaginal immune cell population is unknown (Vojtech et al. 2014). They have been shown to play a direct role in antiviral immune defense (Madison et al. 2014).

Recently, a meta-analysis was conducted on the role of seminal plasma for improved outcomes during in vitro fertilization. The outcome of IVF treatment in patients exposed to seminal plasma near the time of oocyte pickup or embryo transfer was compared to that of controls with no exposure to seminal plasma (a total of 2,204 patients in seven randomized control trials). They found a statistically significant improvement in the clinical pregnancy rate after seminal plasma exposure (RR 1.24, p = 0.003), but no improvement in the ongoing pregnancy/live birth rate (Crawford et al. 2015). However, this topic is a matter of debate. Michael Bedford has pointed out that virgin animals are perfectly good embryo transfer recipients and that many human IVF programs do not use priming with seminal plasma in conjunction with IVF cycles and obtain good fertilization and pregnancy outcomes. He concludes that whereas a nuanced effect of seminal plasma on fertility outcome in humans cannot be dismissed, many experimental and clinical results demonstrate that the presence of seminal plasma is not essential for the transport and survival of spermatozoa in the female tract, for fertilization, or for implantation and embryonic development (Bedford 2015). We review below evidence from in vitro and clinical studies of the effect of seminal plasma on various regions of the human female genital tract.